2013 Tesla Model S: Tire Wear Post-Mortem

September 27, 2013

By now you've read all about the rear tires on our 2013 Tesla Model S and a how a TPMS warning alerted us to a leak. Our P245/35R21 Continentals only had 9,550 miles on them at the time, so we expected something like a nail and a $50 repair. The photo above shows just how wrong we were. And both tires looked pretty much the same.

As soon as I saw this photo I knew the missed rotation had little to do with what went wrong. It was obviously a wheel misalignment issue, and I asked John to make sure to get an alignment printout so I could see what had gone wrong.

Look at the tire again, ignoring, if you can, the mangled inside edge. Concentrate on the face of it. The tire looks to be somewhat evenly worn across its width, with maybe a little less tread depth at the inner wear bar than the outer one.

It appears that things were reasonably OK for a decent amount of time, and then something went horribly wrong. At some point the extreme inside edge began wearing at an entirely different angle, as if the tires suddenly got thrust into some kind of weird pencil sharpener.

The "before" boxes tell the story. The tires ground themselves to bits because the rear wheel alignment went out, on both sides, to more or less the same extent.

But excess negative camber isn't our culprit. Yes, the Tesla Model S's rear camber specification does call for a healthy dose of negative: the range is -1.7 to -2.4 degrees on each side. And it's true our car was at the extreme negative end of that range. The left rear was just inside the limit at -2.3 degrees and the right rear was slightly over limit at -2.5 degrees.

But the extra tenth of a degree on the right rear is not our problem. Do not be overly concerned by this.

Check out the rear toe-in instead. Each side is supposed to be toed-in (angled in at the front — toes in, heels out — when viewed from overhead) anywhere between 0.1 and 0.3 degrees, which makes 0.2 degrees the dead center of the tolerance band. Across both tires, that's a range of 0.2 to 0.6 degrees, with 0.4 degrees being the target you'd shoot for if you were the alignment tech.

But I'm used to measuring and setting toe-in at the racetrack in inches, not degrees. And inches are easier to visualize. So here's a conversion.

Our P245/35R21 tires have a diameter of about 27.8 inches. Across this span, trigonometry says that 0.4 degrees of toe-in is equivalent to 0.19 inches. In tape measure units that's 3/16ths of an inch.

That sounds about right. That's about what I'd have guessed if you'd forced me to invent a rear toe specification at gunpoint.

The Tesla technician told John that the rear toe was the problem, and he was not kidding. Our Tesla's wheels were toed-out instead of toed-in, and not just a little. The left rear was toed-out 0.27 degrees and the right rear was toed-out a whopping 0.46 degrees.

That's 0.73 degrees or almost 3/8ths of an inch of total toe-out. Compared to the toe-in specification, the difference is greater than a half-inch.

This is massive. This is our pencil sharpener for tires. Our wide, low-profile Continentals didn't stand a chance, and no amount of rotation would have prevented it, especially if the alignment went out after the rotation was performed.

How did this happen? I'm still working on that. All I know for sure is the Model S uses an eccentric cam (red) for toe-in adjustment, as shown here in a photo from my 2012 Tesla Model S Suspension Walkaround.

Maybe the nuts weren't tight enough and the cams slipped. Maybe each got a good whack and moved. Or perhaps this bolted joint isn't robust enough to deal with the immense electric motor torque and the rigors of regenerative braking feeding back through in the reverse direction.

Whatever the case, no one's thinking about getting their alignment checked when a car is this new. It's rare to see this sort of thing at such an early stage, and I find it interesting that both sides were affected to a similar extent.

Maybe that's why Tesla replaced these tires on a "goodwill" basis. Why John has found other mentions of this kind of wear when poking around chat rooms. Why a co-worker's dad, a Model S owner, discovered similar unusual wear on his car after we showed him these pictures.

For my part, I'm going to apply paint pen witness marks to the eccentric cams to see if they slip again. And I'll do a quick string-check of the rear toe-in every couple thousand miles to see if I can see signs of any creep there, too.